fix(multiarch): fix 'werf stage image' command panic related to multiarch refactor

Signed-off-by: Timofey Kirillov <timofey.kirillov@flant.com>

Author: distorhead

cmd/werf/run/run.go

@@ -410,6 +410,7 @@ func run(ctx context.Context, containerBackend container_backend.ContainerBacken
 			targetPlatforms = []string{containerBackend.GetDefaultPlatform()}
 		}
 
+		// FIXME(multiarch): specify multiarch manifest here
 		if err := c.FetchLastImageStage(ctx, targetPlatforms[0], imageName); err != nil {
 			return err
 		}

cmd/werf/stage/image/main.go

@@ -207,8 +207,16 @@ func run(ctx context.Context, imageName string) error {
 			return err
 		}
 
+		targetPlatforms, err := c.GetTargetPlatforms()
+		if err != nil {
+			return fmt.Errorf("invalid target platforms: %w", err)
+		}
+		if len(targetPlatforms) == 0 {
+			targetPlatforms = []string{containerBackend.GetDefaultPlatform()}
+		}
+
 		// FIXME(multiarch): specify multiarch manifest here
-		fmt.Println(c.GetImageNameForLastImageStage("", imageName))
+		fmt.Println(c.GetImageNameForLastImageStage(targetPlatforms[0], imageName))
 
 		return nil
 	}); err != nil {